Sea Isle City is a city in Cape May County, in the U.S. state of New Jersey. It is part of the Ocean City metropolitan statistical area and is more generally part of the Delaware Valley or Philadelphia metropolitan area.Sea Isle City was originally incorporated as a borough on May 22, 1882, from portions of Dennis Township, based on the results of a referendum held six days earlier. The borough was reincorporated on March 31, 1890. In March 1907, portions of Dennis Township and Upper Township were annexed to Sea Isle City.